This project uses a prospective double-blind placebo controlled single-case crossover design to assess the role of gonadotropin versus testosterone in the pubertal virilization and sexual behavior of hypogonadal hypopituitary men. The simultaneous assessment of endocrine, somatic (virilization), and behavioral variables during periods of gonadotropin, testosterone, and placebo treatment will result in an unusually comprehensive data base that is both descriptive and quantitative. Consequently, the somatic effects of gonadotropin replacement can be compared to those of testosterone replacement in these men, all of whom have severe gonadotropin deficiency. Similarly, the influence of gonadotropin versus testosterone treatment on the sexual behavior of these men can be examined. In the long run, this information can be used to develop optimal treatment plans for these men as well as help to complete an understanding of the relationship between sex hormones and sexual behavior.